Nonaqueous organic redox flow batteries (NAORFBs) show great promise for grid energy storage but are currently facing key challenges such as high electroactive material cost and low energy density. Herein, we report the electrochemical properties and the potential application of a series of cost-effective electroactive nitrobenzene molecules in NAORFBs. Pairing the low-cost miscible liquid nitrobenzene (NB) with 2,5-di-tert-butyl-1-methoxy-4-(20-methoxyethoxy)-benzene (DBMMB) resulted in a flow battery that provides a high theoretical cell voltage of 2.2 V and a calculated energy density of 192 Wh L −1 . In the charge−discharge testing, this battery delivers a stable cycling capacity retention of 99.5% per cycle over 100 cycles and a 70% energy efficiency at 40 mA cm −2 operation current density, verifying that liquid nitrobenzene is a promising low-cost electroactive anode molecule for NAORFBs.

Aqueous organic redox flow batteries (AORFBs) employing synthetically tailorable organic electroactive compounds have received significant attention for energy storage technologies. There have been many efforts in developing electroactive materials for AORFBs with anion-exchange membranes. On the contrary, electroactive compounds that are compatible with cationexchange membranes in AORFBs are less studied. Here, we report an electroactive 4-carboxylic-2,2,6,6-tetramethylpiperidin-N-oxyl (4-CO 2 Na-TEMPO) molecule for neutral AORFBs. The compound exhibits a good solubility of 1.5 M in an aqueous sodium-based solution, which is 3 times more than that of the pristine 4-hydroxy-2,2,6,6-tetramethylpiperidin-1-oxyl (4-OH-TEMPO). When paired with a 1,10-bis(3-sulfonatopropyl)-4,4′-bipyridinium (SPr) 2 V anolyte, the resulting RFB operating through a cation-exchange membrane achieved an open-circuit voltage of 1.19 V and a high energy density of 14.7 W h L −1 . In the long-term cycling study, the RFB features a stable capacity retention of 99.94% per cycle over 400 cycles with nearly 100% Coulombic efficiency.

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We herein report the synthesis and characterization of a series of ruthenium-substituted Keggin-type heteropolytungstates containing {Ru(II)(NO)}, {Ru(III)(H2O)} or {Ru(IV)Cl} species. Although anionic [PW11O39Ru(II)(NO)](4-) (1) and [PW11O39Ru(III)(H2O)](4-) (2) are known, a new synthetic method for the preparation of (n-Bu4N)4[1] and (n-Bu4N)4[2] is developed in this paper. Treatment of (n-Bu4N)4[XW11O39(Ru[triple bond, length as m-dash]N)] with Me3NO afforded the ruthenium(ii) nitrosyl complex (n-Bu4N)4[1] in almost quantitative yield. Photolysis of (n-Bu4N)4[1] solution in CH3CN/H2O gives (n-Bu4N)4[2], which is readily oxidized by PhICl2 to yield the Ru(IV) complex (n-Bu4N)4[PW11O39Ru(IV)Cl] ((n-Bu4N)4[3]). These complexes are fully characterized by (1)H NMR and (31)P NMR spectroscopy, infrared spectroscopy, cyclic voltammetry, elemental analysis, thermogravimetric-differential thermal analysis, electrospray ionization mass spectrometry (ESI-MS) and X-ray photoelectron spectroscopy (XPS).

We synthesize a covalently linked bipolar molecule 1-(2-(4methoxy-2,5-dimethylphenoxy) ethyl)-1′-methyl-[4,4′-bipyridine]-1,1′-diium hexafluorophosphate (VIODAMB) and apply this new compound to a nonaqueous organic redox flow battery (NAORFB) as both the anolyte and catholyte. We demonstrate that this symmetrical electrolyte can mitigate the cross-contamination issue in the flow battery. The compound exhibits an enhanced solubility of 0.66 M in acetonitrile. The flow cell delivers a capacity retention rate of 80% and an energy efficiency of 85% over 35 cycles at a current density of 1.5 mA cm −2 in the cycling test.

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The operating temperature of vanadium redox flow batteries (VRFBs) affects their performance and reliability. However, previous studies focused on evaluating the effects on the performance of lab-scale single cells, in which electrolyte flow rates and current densities are different from those in stack-scale VRFBs, leading to a lack of guidance for the design of stacks. In this work, we investigate thermal effects on the performance of stack-scale VRFBs. It is found that as the operating temperature increases from 25 to 50°C, the discharge capacity increases by 42%, whereas the energy efficiency increases by 10%, implying that the temperature has greater effects on the discharge capacity than that on the energy efficiency. Additionally, the enhancement effect of temperature on the energy efficiency is gradually weakened with increasing flow rate, while that on the discharge capacity is almost unchanged. Furthermore, the enhancement effect of temperature on energy efficiency increases with the operating current density. Notably, an optimum operating condition of the stack-scale VRFBs is identified with a critical flow rate (2.88 mL min-1 cm-2) at 40°C to achieve a high system efficiency. This work provides guidance for the design of stack-scale VRFBs with high performance and safety.

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